1. Field of the Invention
The present invention relates to a photovoltaic device such as a solar cell and a photosensor, and a manufacturing method of the photovoltaic device.
2. Description of the Related Art
Photoelectric transducers which produce electromotive force upon impingement of light are utilized in various fields. Particularly, with an ever-increasing concern related to environmental problems in recent years, applications of solar cells as clean energy sources are increasingly anticipated.
At the present time, solar cells are mainly grouped into crystalline semiconductor cells using single crystal silicon or polycrystalline silicon, amorphous semiconductor cells using amorphous silicon, and compound semiconductor cells. Among these, amorphous silicon solar cells are expected to dominate the future developments because of their superior features relative to crystal solar cells. This is because a cell having a large area can be easily manufactured and the optical absorption coefficient of the amorphous silicon is so large that the cell can operate in the form of a thin film. However, the amorphous cells are inferior in conversion efficiency to crystalline solar cells.
One of the reasons why solar cells have not been widely used in spite of such expectancy is the high production cost of solar cells. Measures employed for reducing the production cost of solar cells are, for example, as follows:
(1) more efficient utilization of power generating areas, PA1 (2) reduction in the number of electrical connections, with resultant reduction in the cost of connection materials and in the labor cost required for connecting operations, and PA1 (3) reduction in the manufacturing cost of the photoelectric conversion layer, etc. PA1 (1) Due to an increase in the amount of generated currents and in the length of the collecting electrode 710, the resistance loss (i.sup.2 R) is increased and the conversion efficiency is lowered. PA1 (2) When a conductive base plate having a lesser conductivity, such as a stainless base plate, is employed, the resistance loss (i.sup.2 R) is increased and the conversion efficiency is lowered due to the increased path length of the current.
Realizing a solar cell having a large area is essential to achieve improvements in the above areas.
FIGS. 7(a) to 7(c) are schematic views showing a photovoltaic device of the prior art. FIG. 7(a) is a plan view of the photovoltaic device as viewed in the direction facing the light receiving surface, FIG. 7(b) is a sectional view taken along line 7b-7b' in FIG. 7(a), and FIG. 7(c) is a plan view representing two photovoltaic devices connected in series and FIG. 7(d) is a sectional view of the plan view of FIG. 7(c).
The photovoltaic device 700 shown in these Figures is manufactured by successively laminating a lower electrode layer 703 on a base plate 702 made of stainless steel or the like, a semiconductor layer 704 on the lower electrode layer 703, and an upper electrode layer 705 on the semiconductor layer 704. The upper electrode layer 705 is formed of a transparent conductive film such as indium oxide which serves as a reflection preventing means and a current collecting means.
Part of the transparent conductive film is removed along its periphery linearly as indicated by 701 (etching lines) by coating an etching paste, which contains FeCl.sub.3, AlC.sub.3, etc., by screen printing or the like and then heating the paste. The purpose of removing part of the transparent conductive film is to avoid a short circuit between the base plate and the transparent conductive film, that may possibly occur when the photovoltaic device is cut along its outer periphery, and adversely affecting the light conversion property of the photovoltaic device.
On the surface of the photovoltaic device 700, there are formed a plurality of collecting electrodes 707 for efficiently collecting the electric power generated by the device. In order to output the electric power generated in the semiconductor layer without loss, the collecting electrodes 707 are each formed by bonding a metal wire coated with a thin conductive adhesive (e.g., a copper wire coated with a carbon paste) onto the transparent conductive film. The reason for using a copper wire is to reduce current loss by utilizing the high conductivity of copper.
A conductive foil 708 is further disposed as an additional collecting electrode for the collecting electrodes 707. Under the conductive foil 708, an insulating member 709 is provided to ensure insulation between the conductive foil 708 and part of the transparent conductive film along the etching lines where the electrical characteristics are unstable.
In the photovoltaic device thus manufactured, the metal foil 708 and the base plate 702 of stainless steel function as opposite polarity terminals to output the generated electric power.
However, the photovoltaic device having the above structure is insufficient for practical use because the voltage generated by a single photovoltaic cell is too low. To realize a practically usable device, a plurality of cells must be connected in series to obtain an increase in the generated voltage.
FIG. 7(c) shows two photovoltaic devices shown in FIG. 7(a) connected in series. The conductive foil 710 of one photovoltaic device is connected to the base plate 711 of the other adjacent photovoltaic device by using a copper foil (connecting member) 712 so that the two photovoltaic devices are electrically connected in series. A solder containing a flux for stainless steel is used for the connection. After the soldering, the devices are washed by a solvent such as MEK (methyl ethyl ketone) to complete the serial connection.
In an attempt to enlarge the active area of the prior art solar cell explained above, the following problems arise in conversion efficiency with the increased cell area: